The present invention also relates to an optical signal transmission system, comprising at least one transmitter, at least one receiver, and an optical transmission link connecting the transmitter with the receiver, as well as to a method for regenerating optical signals in at least one location along an optical transmission link, in particular in a Dense Wavelength Division Multiplex (DWDM) optical transmission system, comprising the steps of: amplifying the optical signal for compensating optical power losses, and compensating deterministic transmission impairments, in particular chromatic dispersion, of the optical transmission link.
Current (ultra-)long-haul multi-span DWDM optical transmission systems are usually equipped with inline 2R-regenerators (optical fibre amplifiers—OFA) for recovering the optical power level of the DWDM channels in order to improve the link power budget. In some cases, said 2R-regenerators are combined with inline dispersion compensation means in the form of dispersion compensation fibres (DCF) for the adaptation of deterministic residual chromatic dispersion of the link. In the present context, the term “deterministic” generally refers to effects which are foreseeable and predictable with respect to the used transmission wavelengths, the link fibre material, the span length, etc., as is the case with chromatic dispersion. Both functions mentioned above are usually combined in so-called dispersion compensation units (DCU) including an input optical amplifier for compensating the preceding link losses, i.e., power losses induced by preceding fibre spans, an interstage dispersion compensating fibre (DCF), and an output optical amplifier for compensating the DCF losses and for adapting the signal output power level.
However, conventional inline 2R-regenerators are not capable of managing/controlling time variant, e.g. statistically fluctuating, transmission impairments, such as polarisation mode dispersion (PMD) or temperature dependent chromatic dispersion variation on (ultra-)long-haul DWDM links. Such time variant transmission impairments will hereinafter also be referred to as “non-deterministic” dispersion or distortion effects, since they may be foreseeable while generally being unpredictable.
Optical receivers used in transmission systems of the above-mentioned type are usually equipped with forward error correction (FEC) means in order to improve the system margins and to overcome the tight optical signal-to-noise (OSNR) limitations of high bit rate (ultra-)long-haul transmission systems. However, due to the unpredictable, statistically fluctuating signal distortions mentioned above, which may among others be caused by polarisation mode dispersion (PMD), unacceptable error bursts may occur during one FEC frame, thus leading to a system outage in DWDM systems equipped only with conventional 2R-regenerators.